Annular packed-bed filter



Nov. 24, 1970 D. RAMAswAMl ETAL 3,541,762

ANNULAR PACKED-BED FILTER Filed March 18, 1969 De Mabel/Wala' /Pczffafszd azi/7i lef' Jazz/fe l Norway/7.' leva fj E550/:fig

United States Patent Oliice 3,54l,762 Patented Nov. 24, 1970 U.S. Cl. 55--96 1 'Claim ABSTRACT F THE DISCLOSURE Concentric cylindrical screens form an annulus which contains filter material. Gas is passed transversely through the filter material from the outer screen and exits axially upward through the inner screen.

CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

BACKGROUND OF THE INVENTION This invention relates to gas filtration and, in partic- Iular, to filtration of the off-gas from a fiuidized bed.

In one method of reprocessing nuclear fuels, the irradiated fuel is converted to various fiuorides in a fluidized bed by reaction with fiuorine or fiuorine interhalogens. The ofi-gas from the fiuidized bed generally contains some entrained particulate matter which must be filtered from the olf-gas and preferably returned to the fluidized bed. Both doWn-fiow packed-bed and sintered-metal filters are commonly used in conjunction with fiuidized beds.

A down-ow packed-bed filter usually consists of granular inert material placed downstream from the fluidized bed with the off-gas fiowing into the top of the filter and downwardly therethrough to exit from the bottom. Because the down-fiow packed-bed filters use inert filter material, they are operable in high temperatures and in corrosive environments. The chief drawback to the use of down-flow packed-bed filters is that the filtered material cannot be readily returned to the fiuidized bed for further processing. Since this filtered material contains significant amounts of ssionable material, it cannot be rejected to waste; hence, down-fiow packed-bed filters are generally useful only in those processes in which the off-gas contains low concentrations of particulate matter.

Sintered-metal filters are relatively thin, perforate metal sheets. They have successfully been used in fiuidized bed reprocessing of low enriched uranium dioxide fuels. Since most of the filtered material is easily returned to the fiuidized bed by blowback, that is reversing the fiow of gas through the filter, sintered metal filters may be used where the off-gas contains both low and high concentrations of particulate matter. The problem with sintered-metal filters is their reactivity in a corrosive environment at high temperature Where the perforations in the sintered metal provide numerous areas for corrosion. Where the reprocessing scheme is similar to that described in U.S. patent application, Ser. No. 770,145, assigned to the United States Atomic Energy Commission, both high temperatures, in the range of between about 400 C. and about 600 C., and a corrosive environment, such as fiuorine gas or fiuorine-oxygen mixtures, are present and the off-gas contains a large amount of particulate matter. Heretofore, a filter has not been available which will retain its physical integrity in a hot corrosive environment while at the same time provide for easy return of the trapped particulate matter to the fluidized bed.

SUMMARY OF THE INVENTION The present invention entails passing an ofi-gas containing particulate matter from a fluidized bed transversely through the larger of two concentric perforate hollow cylinders which contain inert filter material therebetween. The purified off-gas which enters the innermost perforate cylinder passes at accelerated speeds axially upward through the inner cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS 'lhe single figure is a cross-sectional view of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to the drawing, annular filter 10 abuts cover 11 of fluid-bed reactor 12 and comprises inner and outer perforate hollow cylinders 13 and 14 respectively. An annulus 15 formed therebetween contains inert filter material 16. Filter 10 is closed at the ends by means of bottom plate 17 and top plate 18. Bottom plate 17 is imperforate except for plug 19 which provides means for dumping filter material 16 from annulus 15. Top plate 18 has a plurality of ports 20 in communication with annulus 15, in registry with caps 21 in cover 11, for admitting filter material 16 into annulus 15. A central aperture 22 in top plate 18, in registry with an off-gas exit 23 in cover 11, provides a connection between inner cylinder 13 and an overhead pipe 24. Filter 10 is spaced from a fluidized bed 25 but particulate matter 26 is carried with the off-gas from the bed through the filter along paths a and b.

The off-gas as it passes along paths a and b increases in velocity because an equilibrium exists between the amount of gas entering filter 10 through cylinder 14 and the amount of gas leaving the filter through cylinder 13 while the surface area of cylinder 13 is less than that of cylinder 14. The decrease in surface area is compensated for by an increase in velocity of the off-gas as it passes through filter 10. Increased velocity is important because filtering with inert material such as alumina is by impaction and, generally, the higher the velocity the more efficient the filtration. The particulate matter 26 is trapped both on the outside surface of outer cylinder 14 and on the inert lter material 16. Blowback through filter 10 returns the particulate matter 26 trapped on the outside of cylinder 14 but the patriculate matter trapped on the inert filter material 16 can be returned to bed 25 only by means of plug 19. Usually the inert filter material 16 is the same as the inert material in the bed 25 so dumping will not adversely affect the fluidized bed. Proper correlation of the filter material 16 and the width of annulus 15 will prevent caking of particulate matter 26 on inner cylinder 13 where it is difficult to remove by either blowback or dumping. This is an important consideration where the particulate matter 26 must be recovered for economic reasons.

The following experiments show some of the advantages of an annular filter as compared to a down-fiow packedbed filter. In these experiments the annular filter was constructed from 50 mesh stainless steel screen (31.3 percent open area) with the inner cylinder having a l-inch diameter and the outer cylinder having a 4-inch diameter. Two different batches of alumina were used as inert filter material: one batch ranged in size from -28 to +48 mesh and the other from -48 to +100 mesh. The equipment consisted of a transparent (Lucite) fiuid-bed column 2 inches in diameter and 24 inches high. Both the downfiow packed-bed filter and the annular filter were 4 inches in diameter and about 6 inches in height; they were filled with alumina filter material sized as stated above. However, the annular filter had an inner cylinder one inch in diameter so the filter material was only 11/2 inches wide at any point. The charge to the fiuid bed consisted of 484-100 mesh alumina and -500 mesh nickel fines with particle size for the nickel from 6 to 22 microns. Both filters were almost 100 percent eficient as filters since a downstream absolute filter showed an almost immeasurable amount of nickel fines. Because the 11/2 inches of alumina in the annular filter essential-ly conipletely filtered the particle-containing gas, the inner cylinder 13 did not act as a filter and only an insignificant amount of fines collected thereon. Table I reports data for the down-flow packed-bed filter and Table Il reports data for the annular packed-bed filter.

TABLE I [Fluid-bed charge: 160 g. of -500 mesh nickel lilies; 400 g. of -4SI-100 niesli alumina] Run Run N 0. 1 N o. 2 1

Filter dia. (in.) 4 Alumina, nicsh size -48-l-100 -28+48 Superficial gas velocity th g /sc 0. 250 0. 259 Gas throughput (cfm. per sq./[t. of filter area) 15. 2 15.2 Pressure drop mim-max. (in. of H2O) 16-22. 4 8-10. 2 Total loading of nickel fines 0n filter, g 72.0 8l. 7 Average loading of nickel fines on the filter, kg./sq.

0. 83 0. 036 Depth of penetration of nickel fines (in.), visible.- l/g 1/ Depth of penetration of nickel fines (in),

measured 1 1 TAB LE II [Charge to the fluid-bed column: Alumina 2.0 kg. of 4S-H00 mesh; nickel fines 0.8 kg. of -500 mesh] Run Run No. 3 No. 4

Superficial velocity through the fluid-bed column (ft/sec.) 0. 69 0.69 Alumina charge to annulus filter:

Mesh size 284-48 48|100 Mass, kg 1. 715 1.8 Total gas flow rate through the filter, s.c.f. 2. 02 2.02 Gas throughput for the filter, s.c.f.n1./Sq. it. 11.6 11.6 Run duration, lir 5. 25 5. 83 Mass of nickel fines collected on the filter, g 218. 4 172 Average loading of nickel fines on the filter,

kg./Sq. ft 1. 26 0.99 Mass of nickel fines collected in the filter, g 41. l 48. 7 Mass 0l nickel lines penetrating the filter, mg -1. 0 N1. 0 Pressure drop for fiow of gases through the filter,

inches oi Water 1. 4-2. 0 2. 5-3. 8

A comparison of Runs 1 and 4 and of Runs 2 and 3 shows the advantages of an annular packedbed filter in two of the most important characteristics: average loading on the filter and pressure drop across the filter. As expected, the smaller the filter material the larger the pressure drop, but for Runs 2 and 3 the annular filter showed a 500% lower pressure drop than the down-110W packed-bed filter and for Runs 1 and 4 the annular filter had a 60% lower pressure drop. Additionally, the annular filter uses less alumina filter material although the filtering action is equivalent to the down-fiow pac-kedbed filter.

Further experiments were performed to determine the required width of annulus 15. Using the same apparatus as described above and alumina in the annular filter from -48 to +100 mesh, it was found that a l-inch width of filter material resulted in essentially complete filtration. VFor alumina filter material M28 to +48 mesh, a width of 11/2 inches of filter material was required. These relationships between annulus width and the particle size of the filter material are critical in order to prevent inner cylinder 13 from acting as a filter and collecting particulate matter on its surface. A cake built up on cylinder 13 is difiicult to remove and undesirable; hence, the annulus formed between cylinders 13 and 14 rnust be wide enough, dependent upon the type of filter material contained therein, to provide for essentially complete filtration by the filter material.

It will be understood that the invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claim.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of filtering an off-gas produced by passing a -fiuorine-containing gas upwardly through a fiuidized bed containing alumina as inert material and uranium and plutonium values, said bed being maintained between about 400 C. and about 600 C. comprising passing said off-gas transversely through the `outer of inner and outer 50 mesh screens formed into concentric cylinders separated between one and one and one-half inches, the space therebetween being packed with `--28 to +100 mesh alumina, and containing `28 to +100 mesh alumina'therebetween, passing the filtered off-gas axially upward after -it passes through the inner cylindrical screen, periodically reversing the fiow of gas through the inner and outer concentric screens to return any filtered material caked on the outer screen to the fluidized bed and periodically dumping the alumina contained between' the inner and outer screens into the fluidized bed.

References Cited UNITED STATES PATENTS 2,214,737 9/1940 Dauphinee 55-517 2,928,495 3/1960 Sweet 55-517 3,067,560 12/1962 Parker 55-387 3,164,440 1/1965 Levey 23-353 3,136,149 6/1965 Ayers 55-484 3,296,775 1/1967 Squires 55-387 3,410,055 11/1968 Zenz 55-4517 3,429,669 2/1969 Camozze et a1. 23- 352 2,746,561 5/1956 Beber et ai. 55-302 FRANK W. LUTTER, Primary Examiner e B. NOZICK, Assistant Examiner Us. ci. x11. 

